As is known in the art, it is often desirable to determine the attitude of a spacecraft for payload pointing purposes. Attitude refers to angular orientation with respect to three orthogonal axes. Satellites typically employ attitude sensing apparatus for pointing a payload such as a telescope or antenna to a desired location on the Earth. Conventional attitude sensing apparatus may include a satellite receiver, such as a Global Positioning System (GPS), ground tracking apparatus for locating the satellite ephemerides, and a star tracker for transforming latitude, longitude, and altitude information determined in terms of spacecraft coordinates into a stellar, or orbital frame of reference.
In one satellite attitude sensing system described in U.S. Pat. No. 4,754,280, an array of three satellite receiving antennae are used to sense the attitude of a satellite in accordance with interferometry techniques. A Kalman filter receives the interferometric signals and signals from an inertial measurement unit, generally comprising gyroscopes, and provides periodic optimal estimates of attitude and error rate parameters for each of the gyroscopes. The error rate parameters are used to calibrate the inertial measurement unit output signals in order to compensate for the long term drift associated therewith.
In another attitude sensing system described in U.S. Pat. No. 5,109,346, satellite attitude information is determined by sensing the locations of any two of the Earth, Sun, and Moon. For example, the angular positions of the Sun and Moon with respect to the satellite are measured and are also estimated from ephemeris data. Satellite attitude with respect to a geocentric inertial reference frame is then calculated from the measured and estimated angular positions. In the system described therein, an inertial measurement unit, comprising three orthogonally oriented gyroscopes, is used to update the satellite attitude.